1. Field of the Invention
The present invention relates to orifice meter tubes of the type inserted into pipelines used for custody transfer of gaseous fluids and, specifically, to a device and method for testing the differential pressure measuring passageways of the orifice meter tube to determine the integrity of the passageways.
2. Description of the Prior Art
Metering tubes are commercially available for insertion into pipelines used for custody transfer of gaseous fluids. For instance, a "Flange Neck Superior Meter Tube" is available in 150 to 2,500 pound pressure ratings from Control Specialties, Inc., of Houston, Tex. Such metering tubes are typically provided with either a single or a dual-chamber, flange-neck type orifice fitting. The "orifice fitting" is the portion of the metering tube which holds the orifice plate. The dual chamber orifice fitting design allows orifice plates to be installed and removed conveniently in a pressurized line. The dual-chamber orifice fitting is composed of two independent compartments separated by a stainless steel slide valve. A shaft pinion assembly, included on the orifice fitting, allows the slide valve to be opened and closed for insertion and removal of the orifice plate.
The orifice plate is typically made of stainless steel and contains an orifice which is usually located in the center of the plate. When the plate is installed within the orifice fitting, it is aligned within the metering tube perpendicular to the flow of fluids therein. A differential pressure measuring passageway is provided in the metering tube on either side of the orifice plate so that the pressure drop across the orifice plate can be measured using conventional equipment known to those skilled in the art. By measuring the pressure drop across the plate, the fluid velocity can be calculated.
The differential pressure measuring passageway provided in the casting of the metering tube can have imperfections which result in leakage. This leakage can be caused by porosity in the casting, cracks in the walls of the passageway, and the like. Leakage in the differential pressure measuring passageway results in inaccuracies in the fluid velocity measurement.
The present invention has as its object the provision of a device and method for vacuum testing of the differential pressure measuring passageways of an orifice meter tube to insure the integrity of the passageways.